The present invention relates to an image forming apparatus and a conveyance control method for the image forming apparatus, and in particular, to control in the case of conveying a transfer sheet at high speed.
In an image forming apparatus such as a copying machine, a printer and a facsimile machine, importance has been attached to a period of time from the start of copying processing to output of the first copy sheet (first copy time) and to the number of copy sheets outputted within a unit time.
To improve these functions, it is preferable that a transfer sheet is conveyed continuously without stagnation inside the image forming apparatus.
In the case of an image forming apparatus of an electrophotographic system, therefore, the preferable is an image forming apparatus wherein a transfer sheet passes through a registration roller, a photoreceptor drum and a fixing roller at the ordinary second conveyance speed, and conveyance from a sheet feeding tray and conveyance for sheet-reversing and sheet-refeeding are conducted at the first conveyance speed which is twice the second conveyance speed.
The state of an image forming apparatus as that stated above is shown in FIG. 5. When conducting conveyance by switching to plural conveyance speeds as stated above, speeds are switched by the use of a clutch mechanism, in loop forming rollers 182 arranged at the conveyance path on the upstream side of registration rollers 185 which rotate at constant speed.
Namely, the loop forming roller 182 conveys transfer sheet "p" at the first speed in the case of sheet feeding from a sheet-feeding tray and of conveyance for sheet-reversing and sheet-refeeding, but when the transfer sheet "p" is interposed by both the registration rollers 185 and the loop forming rollers 182 to be conveyed, the loop forming rollers 182 convey the transfer sheet "p" at the second conveyance speed which is the same as that of the registration rollers 185.
In this case, in the loop forming rollers 182 which use plural conveyance speeds for conveying, magnetic clutch C1 is turned on first to transmit torque coming from driving source M1 to the loop forming rollers 182 (FIG. 6(a), (c)), and then, magnetic clutch C2 is turned on to transmit torque coming from driving source M2 to the loop forming rollers 182 (FIG. 6(b), (c)). Incidentally, in the period of time when the magnetic clutch C2 is turned on, the registration rollers are also driven by M2 simultaneously (FIG. 6(d)).
The control of this kind is not limited to an image forming apparatus of an electrophotographic system, but it is a structure which can be applied generally to an image forming apparatus having the structure to convey a recording material to an image recording position, such as that of an ink jet system.
However, when a clutch mechanism is used for switching conveyance speeds, switching time and response time are needed. When an ordinary magnetic clutch is used, there is response dispersion of about 50 milliseconds. Because of existence of this response dispersion, an interval of conveyance for transfer sheets needs to be lengthened, which has checked high speed operations.
When the number of speeds is n, magnetic clutches in quantity of n or (n-1) are required.
When the number of plural conveyance paths such as sheet-feeding cassette and bypass sheet-cassette is m, the number of magnetic clutches required is n.times.m or (n-1).times.m.
For the reasons stated above, there are caused problems of increased number of parts and lowered reliability, and a problem of lowered high speed nature.
When conducting conveyance by switching two speeds as stated above, a conveyance roller arranged immediately before a registration roller which rotates at a constant speed is changed in terms of speed by a clutch mechanism.
However, when the clutch mechanism is used, a period of time for switching and a response time are needed. Further, when conveying a transfer sheet by interposing it with two conveyance rollers, slight disagreement of response time between the two conveyance rollers causes troubles such as overload for a driving source, damage of a transfer sheet and occurrence of abnormal noise both caused by rubbing between the transfer sheet and the conveyance roller.
For preventing these troubles, there has been employed a method to provide a difference between gripping forces of two conveyance rollers when a transfer sheet is interposed by the two conveyance rollers, or a method to give excessive slack to a transfer sheet interposed by two conveyance rollers.
However, giving slack to a transfer sheet between two conveyance rollers creates waste of time for conveyance of the transfer sheet, which has checked high speed operations slightly.
Further, even in the case of the method to provide a difference between gripping forces of two conveyance rollers, it has been difficult for this method to prevent overload for the driving source, damage on a transfer sheet and occurrence of abnormal noise completely.